As I mentioned on other threads, the CB antenna on my car is a 4ft coil-loaded whip. It's exactly the same antenna that everybody uses for off-road stuff, because of its size and ruggedness. While researching why I can't get the wretched thing to work reliably, I came across this quote on a website:

"End-fed antennas always have common-mode currents in some sort of counterpoise, which is the feed line or mast when lacking a suitable counterpoise and isolation at the antenna feedpoint. This does not mean these undesirable currents are always obvious through some sort of terrible problem, or that the antenna will not make contacts, or "make the antenna's owner happy". It means the antenna might not be repeatable or reliable, or have the pattern or performance expected, as a better planned system would have. The benefit is mechanical simplicity and ease of construction, while the cost is repeatable electrical performance. " (my emphasis)

A lack of "repeatable electrical performance" exactly sums up my problem -- SWR can vary from 1:1 to 1:5 just my moving the toolbox in the boot. I don't believe there's any problem with the ground-plane -- at least, not one that could be solved, given that cars are what they are.

I'm starting find that people who use mobile CB very often don't take all that much trouble about their equipment. Most don't know whether they're using AM or FM, so antenna tuning isn't high on the to-do list. I was told by the retailer that I bought the equipment from that probably 80% of purchasers don't make any attempt at all to quantify their system's performance at all. If it doesn't work, they just assume that everybody is too far away, or the weather is too bad, or whatever.

I wonder if the nature of cars -- relatively small metal boxes insulated from the ground -- makes it impractical to design an antenna that is both relatively short, and actually works reliably? Has anybody actually got, or ever used, a CB/11m antenna that was less that 5ft long, and could be tuned to give reasonable SWR that stayed the same from one day to the next?

The clue that moving your tool box makes a difference does provide a useful pointer. The feeder cable is not supposed to have any part in the radiating mode of the aerial, and in fact in many aerial designs, there are design elements employed to try to prevent it. Two things really, any emission from the non-antenna element can be additive or subtractive to the total RF emission, and usually cancellation, or at least partial cancellation occurs - maybe impacting in larger installations in notches in the polar pattern - strange directions where the signal is weaker, that kind of thing. The other thing is that matching the transmitter to the aerial is compromised. Are you matching the aerial, or the aerial and the feeder? In your case, both - as the location of the metallic lump in the boot (the tool box) impacts on the performance.

It suggests the ground plane is to a degree, uncoupled from the aerial. This always happens with mag-mounts, because they float over the top of the ground plane. One thing to check is if the DC resistance from the ground plane to the earth of the radio is a dead short. If it has even a few Ohms of resistance, then RF current is going to start to flow in the feeder cable, making it part of the aerial. It can happen easily on mag mounts on the boot - as rubber seals and greased bearings to make opening and shutting smooth do not ensure a good electrical contact. I've seen examples where the only metal to metal connection is the lock! If the mag mount is on the roof, then good grounding should be much more likely - after all, a metal roof is usually welded to the chassis/body, but you could have poor grounding at the radio end, so the earth /screen of the coax is floating - with the same result. I've used all manner of mag-mounts over the years from CB to ham to business radio, and the only times performance was poor was with grounding issues. Oddly, a direct ground is not actually needed. A handheld, working to a mag mount usually works perfectly fine, because the ground plane in this case is a virtual ground - a ground that functions as one, without any electrical contact. In practice, ground is insulated from electrical ground, and circulating currents don't exist in the feeder cable. Think about marine aerials and fibreglass hulls. From time to time, people use the sacrificial plate fitted to some boats designed to prevent the few metal parts in the water being eaten away by electrolysis, as a ground and often get very unpredicatble RF coverage as a result.

As for solutions for your problem. If the issue is the ground plane is poorly electrically connected to the radio ground, then extra bonding is worth a go - as soon as the tool box movement doesn't change things, you know you have stopped it - or alternatively, you can try a choke. A man sized version of those little lumps you often find in equipment mains cables - computers, power supplies - that kind of thing. If you can find an old, largish loudspeaker, you can make a simple and effective one. Dismantle the speaker back so you can get to the large donut shaped magnet. Be careful because the damn thing will magnetise your tools as quick as a flash. Pull out the ferrite ring, and then take the aerial feeder cable and wrap maybe ten turns of the feeder cable through it, then tape it up, nice and tight. Being magnetic, it will then stick to any bit of convenient metalwork. This works a bit like a shorted transformer, and reduces or often stops the stray RF travelling down the feeder cable screen. If you are desperate, and have an old speaker laying around - well worth giving this a try. These things are mega-magnetic, so will attract all kinds of debris and loose objects so beware.

For some reason I hate the term counterpoise - but it does describe what often happens. Aerial systems need balance and stability for repeatable performance. Almost every aerial type has some kind of balance requirement. The two legs of a half wave dipole, balance at the middle, and even a vertical on a ground plane has some point where balance is possible, if you could somehow detach the ground plane from what it's attached to (difficult of course). Picture your problem as a design with something loose attached to it - that balance is being messed with. You have to restore it.

A mag mount, in the centre of a vehicle roof, with the screen completely floating or 100% connected is stable. I suspect the reason body hole mounts rarely give problems is simply because the metal to metal contact just stops all this faffing around with random current in the feeder. The fact they're rarely central and have a very distorted polar pattern isn't as important to performance as we think!

Thanks for the detailed response. My antenna mount is not magnetic -- it's bolted to a roof bar, with a 10-inch, long 50-amp cable connecting the mount base to one of the bolts that holds the rear tailgate on. The DC resistance between the mount base and the battery negative is zero ohms (well, it's less than 0.1 ohms, anyway, which is the precision of my meter). The DC resistance between the radio chassis and the battery negative is zero ohms. This is why I don't think there's anything else I can do to improve the groundplane, other than buy a new car.

I don't know -- and don't have the tools to find out -- whether the ground bonding is _reactive_, however. I can see how even a few ohms of reactive impedance could arise at 27MHz, that would not show up on a DC meter. But I don't think I can do much about this if it is the case, not without welding, anyway.

But I'm sure you're right (and other people who have commented) : my coax is accidentally part of the antenna. I only have to move my hand around it and watch the SWR needle to see that. I'm sure there is some sort of common-mode current flowing in the coax that arises from ground bonding that is inadequate in some way.

What I'm wondering is whether the antenna I have is simply incapable of behaving nicely, whatever groundplane I have? Is there actually a design that is known to work properly, perhaps with some sort of matching network, and is still robust enough to use on a moving car? I've been to several shops that sell radio equipment, but nobody could recommend anything for mobile CB except the coil-loaded whip I already have.

I can see how I could create a choke that would restrict common-mode currents in the coax. But where would those currents then go? If the ground is already inadequate, for some reason, wouldn't choking of CM current make things even worse?

I don't know for certain, but I've often wondered about roof bars. In essence you have the ground plane separated from the antenna active element, plus they are in effect a loop - you have the ground plane of the roof, then it goes up a few inches, along and then back down. I get your point about the choke - but at least it would prevent the coax joining in the party. Could it be just bad luck that that loop created with the earth bar is causing the issue. I wonder if the design would allow you to isolate the bar from the roof, leaving just your heavy duty earth bond? I'm thinking about a quick try with taped up, or plastic insulated mechanical attachment of the bar itself, so there is only that single earth path - leaving the bars floating? Could be worth seeing of it makes a difference?

I have a Vauxhall Vivaro, and although I didn't get your problems, I had terrible trouble getting a low VSWR on marine band. Distance was pretty short, too - when I used the roof bars - which do clamp very securely. In the end, I drilled a hole and the performance improved astoundingly. The performance is good enough that I can use the 157 cut whip on 2m and business VHF with hardly any drop in range compared to the proper whip for these bands.

I'm just wondering if 'detaching' the ground plane from immediately below the feed point reduces the efficiency, and is adding that inductive/capacitive unwanted component. I'm not actually sure how you could measure (or even detect it). I wonder if an antenna analyser would show anything interesting. On one project I did where the ground plane was distorted by obstacles in the only mounting position - I had to bend the radials forming the ground plane, up down, left and right - not by much, but perhaps an inch or two away from their proper position, because there was a steel vertical beam in the way. The display on the analyser was very unusual - instead of the gradual descent to the low VSWR point then a gradual rise again, I had multiple little troughs, each one gradually getting lower - imagine a series of w curves, gradually dropping then increasing again. The steel coupling with the ground plane caused it - but I don't know if this is the same issue you are getting? Is it worth trying a mag mount design to rule out any other cause?

Thanks. I'm going to try to borrow a mag-mount and see what effect that has. It's tricky because the roof is ridged, so mag-mounts don't stick very well. Presumably that means that the capacitive coupling will be less-than-ideal as well, no? The roof ridges are about a half-inch high. But it's certainly worth a try.

To be honest, I don't have the slightest problem with drilling a hole in the roof if I could be sure it would help. But I'd be gutted if I did that, and it didn't help.

I also have wondered about the effect of the roof bars. I've seen other people mount their CB's the same way, and I bought a mount that is sold specifically for that purpose. So I sort-of assumed it would be OK. But it does put the antenna base a couple of inches above the ground-plane and, for sure, it does create a sort of "dual ground plane" set-up, with a small ground-plane on top of a larger one. On the face of it, I wouldn't have expected this to be a huge problem, but I'm coming to understand that almost nothing about RF is predictable in common-sense electrical engineering terms

Anyway, thanks; you've given me some ideas which I can try, when the temperature gets back above zero.

lars wrote: I also have wondered about the effect of the roof bars. I've seen other people mount their CB's the same way, and I bought a mount that is sold specifically for that purpose. So I sort-of assumed it would be OK. But it does put the antenna base a couple of inches above the ground-plane and, for sure, it does create a sort of "dual ground plane" set-up, with a small ground-plane on top of a larger one. On the face of it, I wouldn't have expected this to be a huge problem, but I'm coming to understand that almost nothing about RF is predictable in common-sense electrical engineering terms

The problem is most CB'ers know next to nothing about radio, as they don't need to as it's not part of the hobby. And often what is known is learnt via hearsay & from others who know nothing.A roof bar mounted antenna at say 2 meters is OK. It's not great, but it's OK, as it needs very little in the way of a groundplane.But at 27 mhz you are on the boarder of where a magmount will work & where a good counterpoise is paramount & a roofbar is simply never going to work effectively unless it has added metal, as it's just too small. Many probably get away with it, but that does not mean it works. Just that it works, sort of & with issues that they may lack the understanding to know about or spot. A few years ago I was looking for a decent antenna for 10 meters, thats 28 to 29.7mhz. The SSB portion of the band is around 28.3 to 28 5 mhz & that I could have covered easily with a CB antenna, but finding a well designed CB antenna is hard, there are some around, but most are poorly designed & much more of a compromise than they need to be. In the end I brought a amateur radio multi band antenna, which cost a similar price to a decent CB antenna & I forgot about using anything single band on 10 meters.

Thanks. I don't the problem is whether my roof bar is an adequate ground-plane, since it's very stoutly connected to the roof, which is where I understand the ground-plane to start. If I understand G4RMT correctly, the problem is whether the roof-bar mounting is actually preventing the car body being an adequate ground-plane. I think this is something I can check. I don't understand why a problem would arise with this kind of mounting (given that the bar is connected to the roof), but that I don't understand anything about RF.

I would cheerfully buy a better antenna, if I could find one, and even if I had to tweak it to suit the CB band. The problem is that (so far as I can tell) amateur 10m/11m antennas tend to be either very long, or rigid, or both, which would be a real problem in woodland, as the antenna needs to survive low branches. But I'm certainly willing to consider anything that stands a chance of working.

lars wrote: I don't understand why a problem would arise with this kind of mounting (given that the bar is connected to the roof), but that I don't understand anything about RF.

It is NOT connected to the roof, it is a small length of something that may or may not be ferrous metal sitting on top of the roof, but normally separated from it, by rubber pads that are there to stop damage. Normally roof bars are at the very least coated in something & often are made of something other than metal. Also, very often body panels on modern vehicles are electrically separate from each other & the chassis. Bonding panels can create rust issues in some modern vehicles.

lars wrote: it's not part of the hobby. And often what is known is learnt via hearsay & from others who know nothing.A roof bar mounted antenna at say 2 meters is OK. It's not great, but it's OK, as it needs very little in the way of a groundplane.But at 27 mhz you are on the boarder of where a magmount will work & where a good counterpoise is paramount & a roofbar is simply never going to work effectively unless it has added metal, as it's just too small. Many probably get away with it, but that does not mean it works. Just that it works, sort of & with issues that they may lack the understanding to know about or spot. ............................

This explains why I get poor reception from nearby mobiles, not helpful when trying to get an antenna spot on!